The long-term objectives of this project are to develop the best and the safest method of pain control for the surgical patients. In early 1970s, general anesthetics and various opioid analgesics were found to exert the direct antinociceptive effects at the spinal cord level, in addition to their supraspinal effects. This was initially demonstrated by an electrophysiological study, examining the nociceptive single neuron activity modified by analgesics and anesthetics. In the mid 1970s, spinal opioid analgesia was demonstrated by behavioral studies such as tail-flick response and hot plate response. The results of these investigations led to the clinical use of opioid in the production of spinal analgesia in man, one of the major developments in pain control during the past decade. These electrophysiological and behavioral experiments are the two major methods of preclinical studies to evaluate the spinal analgesia. While the electrophysiological experiment utilizing the single neuronal unit recording techniques has demonstrated clear evidence of opioid receptor binding and activity at the spinal level, to what extent the suppression of single neuronal activity contribute to the overall analgesic action of the drug has not been demonstrated. While the results of the behavioral study is not able to provide underlying mechanisms of action of the analgesic drugs, the electrophysiological study is able to do so. Therefore, the specific aims of this proposed investigation are to compare the results of the electrophysiological experiment with that of the behavioral experiment under identical experimental set-ups. This will enable us to make meaningful comparison, to demonstrate the role of spinal neurons in overall pain processing, and to elucidate the mechanisms and sites of action of analgesic drugs acting at the spinal level. We will also conduct comparative investigation of spinal analgesia between somatic and visceral pain processing, aiming at the development of better pain control in these two important clinical pain entities.